2.1 Autoimmune Diseases
Autoimmune diseases are caused when the body's immune system, which normally defends the body against bacteria, viruses and other infective agents, attacks “self” tissue, cells and organs. The mobilization of the immune system against such “self” targets is termed autoimmunity. Although some autoimmunity is present in every individual, rigid control systems suppress the self-recognizing cells of the immune system to an extent that the autoimmunity is normally asymptomatic. Disease states arise when there is some interruption in the control system, allowing the autoimmune cells to escape suppression, or when there is some change in a target tissue such that it is no longer recognized as self. The mechanisms underlying these changes are not well understood, but have been theorized to be the result of aberrant immune stimulation in genetically predisposed individuals.
Autoimmune diseases can be organ specific or systemic and are provoked by differing pathogenic mechanisms. Organ specific autoimmunization is characterized by tolerance and suppression within the T cell compartment, aberrant expression of major-histocompatibility complex (MHC) antigens, antigenic mimicry and allelic variations in MHC genes. Systemic autoimmune diseases involve polyclonal B cell activation and abnormalities of immunoregulatory T cells, T cell receptors and MHC genes. Examples of organ specific autoimmune diseases are diabetes, cutaneous psoriasis, ulcerative colitis, hyperthyroidism, autoimmune adrenal insufficiency, hemolytic anemia, multiple sclerosis and rheumatic carditis. Representative systemic autoimmune diseases are systemic lupus, erythematosus, rheumatoid arthritis, psoriatic arthritis, Sjogren's syndrome polymyositis, dermatomyositis and scleroderma.
Also, while not an autoimmune disorder, organ transplant recipients often experience similar symptoms and require therapies similar to autoimmune patients. Immune system attacks on the transplanted organs can lead to organ failure or more serious systemic complications, i.e. bone-marrow transplant recipients and graft-vs.-host disease (GVHD).
There is a clear need for improved strategies to treat autoimmune disorders and/or modulate immune response. Currently, immune system disorders are treated with immunosuppressive agents such as cortisone, aspirin derivatives, hydroxychloroquine, methotrexate, azathioprine,cyclophsophamide and various biologics such as anti TNF antibodies, or combinations thereof. The treatments are varyingly successful, depending on the individual patient and disorder. However, a dilemma in the use of such general immunosuppressive therapies arises in that the greater the immune-suppression, and thus the potential for successful treatment of the autoimmune disorder, the more at-risk the patient becomes for developing opportunistic infections. Further, due to the compromised nature of the patient's immune system, even a minor infection can rapidly become of serious concern.
2.2 Diabetes
One of the most common autoimmune diseases is type 1 diabetes, also known as juvenile diabetes or insulin-dependent diabetes mellitus. It affects 15 million people in the United States with an estimated additional 12 million people who are currently asymptomatic, and thus are unaware that they have this disease. Diabetes is caused by an autoimmune response in which the insulin producing β-cells of the pancreas (also known as islet cells) are gradually destroyed. The early stage of the disease, termed insulitis, is characterized by infiltration of leukocytes into the pancreas and is associated with both pancreatic inflammation and the release of anti-β-cell cytotoxic antibodies. As the disease progresses, the injured tissue may also attract lymphocytes, causing yet further damage to the β-cells. Also, subsequent general activation of lymphocytes, for example in response to viral infection, food allergy, chemical, or stress, may result in yet more islet cells being destroyed. Early stages of the disease are often overlooked or misdiagnosed as clinical symptoms of diabetes typically manifest only after about 80% of the β-cells have been destroyed. Once symptoms occur, the type-1 diabetic is normally insulin dependent for life. The dysregulation of blood-glucose levels associated with diabetes can lead to blindness, kidney failure, nerve damage and is a major contributing factor in the etiology of stroke, coronary heart disease and other blood vessel disorders.
2.3 T Cell Functionality in Diabetes and Other Autoimmune Disorders
Destruction of β-cells in diabetes, or of the target cells of other autoimmune disorders, is believed largely mediated by cytotoxic T-lymphocytes (CTLs—also known as CD8+T cells) that specifically recognize antigenic, target cell derived peptides. CTLs, as well as other types of T cells, recognize these antigenic peptides through their specific T cell receptor (TcR). Unlike antibodies which recognize soluble whole foreign proteins as antigen, the TcR instead interacts with small peptidic antigens presented only in complex with major histocompatibility complex (MHC) proteins.
Most cells of the body express MHC molecules of various classes on their surface and, depending on the class of MHC expressed, will present either soluble antigens, those dispersed within the lymph and/or circulatory systems, or fragments of their cytoplasmic proteins. MHC molecules (called human leukocyte antigens or HLA in humans) and TcRs are extremely polymorphic, each clonal variation recognizing and binding to a single peptidic sequence, or set of similar peptidic analogs. Apart from cells specific to the immune system, i.e. B cells and T cells, cells of the body express multiple variants of the MHC molecule, each variant binding to a different peptide sequence. In contrast, during maturation, B and T cells lose the ability to express multiple variants of MHC and TcR, respectively. Mature T cells, therefore, will express only one of the possible variants of the TcR and will thus recognize/bind a single MHC/antigen complex.
Binding of a TcR to a MHC/antigen complex releases an intracellular signal cascade within the T cell, termed activation, which results in clonal proliferation of the T cell and class-specific T cell responses. For example, in CTLs the response to activation also includes the release of cytotoxic enzymes that result in apoptosis/destruction of the target cell.
2.4 Modulation of T cell Activation by Monoclonal Antibodies
The finding that autoimmune diseases are at least partially caused by aberrant T cell action has lead to the investigation of therapies that either eliminate problematic T cell clones (those expressing TcRs against self antigens) or selectively reduce undesired T cell activity/activation. T cell activation due to TcR binding is, however, an unexpectedly complex phenomenon due to the participation of a variety of cell surface molecules expressed on the responding T cell population (Billadeau et al., 2002, J. Clin. Invest. 109:161-168; Weiss, 1990, J. Clin. Invest. 86:1015-1022; Leo et al., 1987, PNAS 84:1374-1378; Weiss et al., 1984, PNAS 81:4169-4173; Hoffman et al., 1985, J. Immunol. 135:5-8).
Targeted therapies directed against general T cell activation were problematic in that the TcR is composed of a disulfide-linked heterodimer, containing two clonally distributed, integral membrane glycoprotein chains, α and β, or γ and δ. Most of the research in modulation of T cell activation was done in connection with improving immune suppression in organ transplant recipients. One of the first clinically successful methods of selectively reducing T cell activation was the use of monoclonal antibodies. U.S. Pat. No. 4,658,019, describes a novel hybridoma (designated OKT3, ATCC Accession No. CRL-8001) which is capable of producing a murine monoclonal antibody against an antigen found on essentially all normal human peripheral T cells. Binding of OKT3 to T cells in vivo produces pronounced, reversible immunosuppression. OKT3 was found to recognize an epitope on the ε-subunit within the human CD3 complex (Salmeron et al., 1991, J. Immunol. 147:3047-3052; Transy et al., 1989, Eur. J. Immunol. 19:947-950; see also, U.S. Pat. No. 4,658,019). The CD3 complex (also known as T3) is comprised of low molecular weight invariant proteins, which non-covalently associate with the TcR (Samelson et al., 1985, Cell 43:223-231). The CD3 structures are thought to represent accessory molecules that may be the transducing elements of activation signals initiated upon binding of the TcR α-β to its ligand.
OKT3 possesses potent T cell activating and suppressive properties (Landgren, 1982; Van Seventer, 1987; Weiss, 1986). Fc receptor-mediated cross-linking of TcR-bound anti-CD3 mAb results in T cell activation marker expression, and proliferation (Weiss et al., 1986, Ann. Rev. Immunol. 4:593). Similarly, in vivo administration of OKT3 results in both T cell activation and suppression of immune responses (Ellenhom et al., 1992, Transplantation; Chatenoud, 1990, Transplantation 49:697). Repeated daily administration of OKT3 results in profound immunosuppression, and provides effective treatment of rejection following renal transplantation (Thistlethwaite, 1984, Transplantation 38:695).
The use of therapeutic mAbs, including for example OKT3, is limited by problems of “first dose” side effects, ranging from mild flu-like symptoms to severe toxicity. The first dose side effects are believed to be caused by cytokine production stimulated by T cell activation. It has been shown that the activating properties of mAbs result from TcR cross-linking mediated by the mAb bound to T cells (via its F(ab′)2 portion) and to FcγR-bearing cells via its Fc portion) (Palacios et al., 1985, Eur. J. Immunol. 15:645-651; Ceuppens et al., 1985, J. Immunol. 134:1498-1502; Kan et al., 1986, Cell Immunol. 98:181-185). For example, the use of OKT3 was found to trigger activation of mAb-bound T cells and FcγR-bearing cells prior to achieving immune suppression, resulting in a massive systemic release of cytokines (Abramowicz, 1989, Transplantation 47:P606; Chatenoud, 1989). Reported side effects of OKT3 therapy include flu-like symptoms, respiratory distress, neurological symptoms, and acute tubular necrosis that may follow the first and sometimes the second injection of the mAb (Abramowicz, 1989; Chatenoud, 1989; Toussaint, 1989, Transplantation 48:524; Thistlethwaite, 1988, Am. J. Kid. Dis. 11:112; Goldman, 1990, Transplantation 50:148).
Data obtained using experimental models in chimpanzees and mice have suggested that preventing or neutralizing the cellular activation induced by anti-CD3 mAbs reduces the toxicity of these agents (Parleviet, 1990, Transplantation 50:889; Rao, 1991, Transplantion 52:691; Alegre, Eur. J. Immunol., 1990, 20:707; Alegre, 1990, Transplant Proc. 22:1920; Alegre, Transplantation. 52:674, 1991; Alegre, J. Immun.; 1991; Ferran, 1990, Transplantation, 50:642). In addition, previous results reported in mice using F(ab′)2 fragments of 145-2C11, a hamster anti-mouse CD3, have suggested that, in the absence of FcγR binding and cellular activation, anti-CD3 mAbs retain at least some immunosuppressive properties in vivo (Hirsch, Transplant Proc., 1991, 23:270; Hirsch, J. Immunol., 1991, 147:2088).
2.5 Immunosuppressive Monoclonal Antibodies Exhibiting Reduced T Cell Activation
U.S. Pat. No. 6,491,916, U.S. Pat. application Pub. No. 2005/0064514 and U.S. Pat. application Pub. No. 2005/0037000 describe the modification of the Fc regions of immunoglobulins such that the variant molecules exhibit enhanced or reduced binding to various Fc receptors when compared to immunoglobulins with wild type Fc domains. In particular the patents/applications describe modifications to the Fc regions of IgG antibodies such that the affinity for the FcγR is selectively enhanced or reduced. By tailoring the affinity for activating or suppressive Fc receptors, the specific immune response elicited by the therapeutic mAb may be more selectively controlled. For example, mutations in the CH2 portion of a humanized OKT3 IgG4 have been identified (P234A and L235A) that significantly reduced binding of the mAb to human and murine FcγRI and II and lead to a markedly reduced activating phenotype in vitro (Alegre et al., 1992, 8th International Congress of Immunology 23-28; Alegre et al., 1994, Transplantation 57: 1537-1543; Xu et al., 2000, Cell Immunol. 200:16-26). Importantly, this variant mAb retained the capacity to induce TcR modulation and immunosupression (Xu et al., 2000).